1. Field of the Invention
The invention generally relates to powder coating application systems, and more particularly to apparatus and methods that fluidize and deliver powder and use electrostatic charge to assist in adhering powder coatings to work pieces.
2. Background of the Invention
Electrostatic powder spray guns are known for use in applying protective coatings to industrial and commercial products. Typically, a finely divided powder is transported through a spray head while entrained in an air or gaseous stream that is discharged from the spray head. The entrained powder is directed by the spray head toward a target article or work piece and is drawn toward the article or work piece by opposite electrostatic charges (at least effectively) on the powder and on the work piece. After the work piece has been coated with the charged powder, the article is heated. The powder melts and flows together to bond and form a more permanently adhered coating as it cools.
Electrostatic powder spray guns as described are well known in the art. Examples are disclosed in U.S. Pat. No. 4,079,894--Harjar, et al.; U.S. Pat. No. 4,143,819--Hastings; U.S. Pat. No. 4,380,320--Hollstein, et al.; U.S. Pat. No. 4,529,131--Rutz; U.S. Pat. No. 4,543,274--Mulder; U.S. Pat. No. 4,580,727--Moos; U.S. Pat. No. 4,630,777--Hollstein, et al.; U.S. Pat. No. 4,706,890--Talacko; U.S. Pat. No. 4,747,546--Talacko; 4,886,215--Ruud; U.S. Pat. No. 5,056,720--Crum, et al.; U.S. Pat. No. 5,395,046--Knobbe, et al.; U.S. Pat. No. 5,402,940--Haller, et al.; and U.S. Pat. No. 5,622,313--Lader, et al. These patents teach various apparatus, methods, and systems for the electrostatic application of powder coatings to a work piece, and are hereby incorporated herein by reference.
Conventional electrostatic powder coating apparatus typically comprise a spray gun that charges the powder in one of two ways. In one type of device, the gun has a high voltage charging electrode which produces a corona that charges the powder as powder particles move through the corona. Voltages in the range from about 30 to about 100 kilovolts (kV) are typically applied to the electrode in this type of spray gun. Gas propellant pressures in the range of 30 to 70 pounds per square inch (psi) or more are often required as well. While such guns are suitable for many industrial applications, they can be difficult and/or expensive to operate. Additionally, the high voltages and gas pressures required for satisfactory operation make these devices potentially dangerous, particularly for casual users such as small shops and home hobbyists.
In a second type of known device, the gun charges the powder by friction, i.e., triboelectrically, the electrostatic effect produced by rubbing a nonconductive article such as an inflated rubber balloon against certain fabrics. In triboelectric electrostatic spray guns, contact surfaces along the discharge path of the spray gun are constructed from an electrically insulating material, typically a polymer. During spraying, the finely divided powder particles are caused to impact the contact surfaces numerous times and thereby become frictionally charged. High voltage is not required, but propellant gas pressures in the range of 30 to 70 psi or more are often required to produce the necessary frictional contact to effectively charge the sprayed powder. For example, in U.S. Pat. No. 4,886,215--Ruud, also incorporated, a hand-held powder spray gun is disclosed that includes a casing enclosing a plurality of helical charging ducts mounted on parallel cores. The ducts receive the powder from a diffuser mounted in a depending gun handle and discharge charged powder toward a nozzle mounted at one end of the casing. The helical paths increase the length of the ducts and the extent of frictional contact as compared to a straighter path.
An undesirable consequence of the triboelectric interaction between the powder and the contact surfaces of the gun is the creation of a charge on the contact surfaces that is opposite in polarity to that imparted to the powder particles. This effect reduces the efficiency of the gun, and has resulted in various attempts in the art to control or reduce the build up of charge on the gun's contact surfaces. Thus, in U.S. Pat. No. 4,706,890--Talacko, a spray gun is disclosed which contains a discharge electrode disposed entirely outside the region where the powder flows and in electrical communication with the gas flowing in a gas conduit. The powder is accelerated, by pressurized gas, through an annularly shaped and axially extending channel in which the coating material is electrically charged by friction. The channel is defined between inner and outer members formed from different electrically insulating materials. Unwanted charge that builds up on the inner and outer members is discharged to the electrode through the flowing gas.
In U.S. Pat. No. 5,402,940--Haller, et al., a triboelectric powder spray gun is disclosed including a diffuser for mixing powder with a conveying gas, a charging portion located downstream from the diffuser, and a spray head at the outlet of the charging portion for dispensing the charged powder. Haller discloses contact surfaces capable of triboelectrically charging the powder as it transits the gun. Haller recognizes that the electrically insulating contact surfaces become charged opposite in polarity to the charge of the transiting powder and therefore provides a grounding ring, disposed in electrical contact with the charging surfaces, for providing a conduction path to ground for the charge that builds up on the contact surfaces.
In U.S. Pat. No. 5,622,313--Lader, et al., a triboelectric powder spray gun is disclosed in which a charge is imparted triboelectrically to the powder by repeated impacts of the powder with internal contact surfaces formed from electrically insulating materials such as PTFE or polyamide. Lader et al. increase the charging effectiveness of their gun by using an electrode disposed within the gun barrel to produce a corona treatment of the contact surfaces to discharge the contact surfaces and eliminate the need for adjacent ground pathways within the powder flow path.
There is an unsatisfied need for a powder spray gun that can be operated without the application of any voltages, or at relatively low applied voltages, for charging the powder. There is also a need for a powder spray gun that may be operated at relatively low air pressures. There is also a need for a gun mounted powder container that holds a sufficient amount of powder and fluidizes the powder internal to the cup. These improvements would render the spray gun inexpensive, safe, portable and would allow for quick change of powder color. Such a spray gun would be especially suitable for use by small shops and home hobbyists.